1. Field of the Invention
The present invention generally relates to a high-frequency amplifier suitable for use in radio communication equipment such as portable telephones and cordless telephones and, more particularly, to a high-frequency amplifier which performs matching corresponding to each of a plurality of high-frequency signals of different frequencies and that these high-frequency signals are amplified by a single amplification circuit.
2. Description of the Related Art
Hitherto, in mobile (radio) communication equipment such as portable telephones and cordless telephones, radio waves of a corresponding dedicated frequency band (namely, an exclusive or private frequency band) have been used. Therefore, only a high-frequency amplifier for amplifying high-frequency radio signals in the exclusive frequency band is needed to be provided in a transmitting device (or transmitter) incorporated in a portable telephone or the like.
Hereinafter, a conventional high-frequency amplifier will be described with reference to FIG. 10.
In FIG. 10, reference numeral 1 designates the conventional high-frequency amplifier. This high-frequency amplifier 1 is provided in a transmitter incorporated in, for example, a portable telephone set and is a power amplifier dedicated to amplifying high-frequency signals in a dedicated frequency band, which are transmitted as radio waves from this portable telephone. For instance, the frequencies of the dedicated frequency band range from 890 MHz to 915 MHz, the center frequency thereof using 902.5 MHz, and the high-frequency amplifier 1 may be suitable for amplifying only high-frequency signals of such a frequency band.
Reference numeral 2 denotes an input terminal of the high-frequency amplifier 1. This input terminal 2 is connected to a transmission signal generating circuit (not shown) incorporated in the portable telephone, which synthesizes a signal from a speech signal and a carrier signal and outputs transmission input signals (namely, high-frequency input signals), to this input terminal 2.
Reference numeral 3 designates an input matching circuit connected to the input terminal 2 through a coupling capacitor 4. This input matching circuit 3 is configured by connecting a coil 5, which serves as an inductive device or element, to a capacitor 6, which serves as a capacitive device or element, in an L-shaped configuration, as illustrated in FIG. 10. Parameters such as the inductance of the coil 5 and the capacitance of the capacitor 6 are set in such a manner that matching is achieved corresponding to the center frequency (namely, 902.5 MHz) of the transmission input signal. Namely, a reflection coefficient corresponding to the center frequency of the transmission input signal is optimized. Moreover, the parameters of the coil 5 and the capacitor 6 are set in such a way that the input return loss of the high-frequency amplifier 1 is minimized.
Reference numeral 7 designates a power amplification circuit connected to the output terminal of the input matching circuit 3. This power amplification circuit 7 comprises a field effect transistor (hereunder referred to as "FET") 8, which is operative to perform power amplification, and a bias circuit (not shown). Further, the power amplification circuit 7 amplifies the transmission input signal enough so that the input signal can be transmitted by the portable telephone, for example, to 1 W or so. The amplified transmission output signals are outputted to an output matching circuit 9 (to be described later).
Reference numeral 9 denotes the output matching circuit provided at an output side of the power amplification circuit 7. This output matching circuit 9 is constituted by connecting a coil 10 to a capacitor 11 in an L-shaped configuration, almost the same as the input matching circuit 3. Parameters such as the inductance of the coil 10 and the capacitance of the capacitor 11 are set in such a manner that matching is achieved corresponding to the center frequency (namely, 902.5 MHz) of the transmission output signal. Namely, by taking the output characteristics, the bias conditions and gain of the power amplification circuit 7 into consideration, each of the parameters of the coil 10 and the capacitor 11 is set in such a way that the reflection coefficient corresponds to the center frequency of the aforementioned transmission output signal.
Reference numeral 12 designates a coupling capacitor for eliminating a bias current. Reference numeral 13 denotes an output terminal of the high-frequency amplifier 1. This output terminal 13 is connected to an antenna attached to the portable telephone.
In the conventional high-frequency amplifier 1 configured in this manner, the input matching circuit 3 is set in such way that the matching corresponds to the transmission input signal, whose center frequency is 902.5 MHz.
However, in some cases, the transmission frequency band of a portable telephone must vary with the area, in which the portable telephone is used. For example, the frequency band may range from 890 MHz to 915 MHz in a certain area, and may range from 1430 MHz to 1450 MHz in another area. Thus, a portable telephone of the common type, by which calls can be made in a plurality of areas, must be able to transmit in at least two different frequency bands.
Thus, as shown in FIG. 11, another portable telephone of the common type has a parallel connection of a first high-frequency amplifier 21, by which matching is accomplished corresponding to the frequency band of a certain area, and a second high-frequency amplifier 22, by which matching is accomplished corresponding to the frequency band used in another area. Changeover switches 23 are provided at the input side or terminal and the output side or terminal of each of the high-frequency amplifiers 21 and 22 so as to amplify transmission signals in two different frequency bands.
This amplifier, however, is required to have two power amplification circuits 21 and 22, of the same configuration, in parallel, as well as the changeover switches 23. Consequently, the size of a circuit board, on which these circuits and switches are mounted, increases. Moreover, it is difficult to reduce the size and weight of the amplifier. Additionally, the number of parts or components increases. This results in an increase in the cost of the amplifier.